The present invention relates generally to integrated circuits. More particularly, the present invention relates to an output control circuit for use to reduce power or ground bounce noise, or both.
Digital logic circuits are widely used in the areas of electronics and computer-type equipment. One such use of digital logic circuits is to provide an interface function between one integrated circuit chip and another integrated circuit chip. An output buffer is an important component for performing this interface function. The output buffer generates, when enabled, an output signal which is a function of a data signal received from internal logical circuitry of the integrated circuit.
An output buffer circuit typically uses a pull-up transistor connected in series between first and second power supply terminals. The first power supply terminal maybe supplied with a positive potential which is connected to a power supply potential node. The second power supply terminal may be supplied with a ground potential, which is connected to a ground potential node. The connection point of the pull-up and pull-down field-effect transistors is further joined to an output terminal.
Dependent upon the logic state of the data input signal (and an output enable signal for a tristateable output buffer), either the pull-up or pull-down transistor is quickly turned on and the other one of them is turned off. Such rapid switching on and off the pull-up and pull-down transistors causes sudden surges of current creating what is commonly known as current spikes. These current spikes will flow through the impedance and inductive components of power supply lines so as to cause inductive noise at the internal power supply potential and the internal ground potential nodes of the output buffer. In particular, when the pull-down transistor is quickly turned on a large instantaneous current cooperates with the line inductance to pull up the internal ground potential which is defined as "ground bounce noise." The line inductance and resistance may be a parasitic artifact from the bonding wires and lead frame.
Similarly, when the pull-up transistor is turned on too quickly, there may be "power bounce noise" on the power potential.
Therefore, it is desirable to provide an output current control circuit for use with an output buffer which reduces ground bounce and power bounce noise, but yet provides high speed of operation.